1. Field of the Invention
The present invention relates to a method for eliminating a blocking effect in a compressed video signal, and more particularly, to a method for eliminating a blocking effect using a loop/post filter in H.26L moving picture compression method.
2. Description of the Background Art
Generally, a hybrid MC-DCT technique employing a basic algorithm, such as H.261, H.263, MPEG1, MPEG4, etc., is used as a method for compressing a moving picture. In the hybrid MC-DCT technique, spatial and temporal redundancy of an video signal to be compressed is removed.
FIG. 1 is a block diagram illustrating an video encoding unit, which including a first switching unit 2 for controlling an input video signal; a DCT unit 3 for performing a discrete cosine transform operation of the video signal; a quantization unit 4 for quantizing the discrete cosine transform signal to output a quantized DCT coeffcient; an encoding controller unit 5 for controlling the first switching unit 2 and the quantization unit 4 and outputting INTRA/INTER information(p=mtype;flag for INTRA/INTER), transmission information(t;flag for transmitted or not), and quantization information(qz;Qp; quantizer indication); an inverse quantization unit 6 for inversely quantizing the value outputted from the quantization unit 4; an inverse DCT unit 7 for performing an inverse DCT operation of the inversely quantized signal to restore the same; a loop filter 9 for compensating the motion picture by searching for the motion vector of the restored signal; and an video memory 10 for storing the compensated video signal.
The operation of the thusly constructed video encoding unit will be described as follows.
When an video signal is inputted from the outside, the inputted video signal is subtracted from a differential signal stored in the video memory 10, said subtracted video signal being inputted into the first switching unit 2. The first switching unit 2 switches the appropriate signal of the subtracted video signal and the differential signal stored in the video memory to input the same to the DCT unit 3, the DCT unit 3 performing a discrete cosine transform operation of the inputted signal to output the same to the quantization unit 4. The quantization unit 4 quantizes the inputted signal to output a quantized DCT coefficient(q). The quantized signal can be adapted to the pixel value of a still picture or moving picture, or transform coefficients of a pixel.
At this time, the encoding controller 5 controls an video signal input switching process of the first switching unit 2 and a quantization process of the quantization unit 4 to output INTRA/INTER information(p=mtype;flag for INTRA/INTER), transmission information(t;flag for transmitted or not), and quantization information(qz;Qp; quantizer indication).
Meanwhile, the signal outputted from the quantization unit is inversely quantized in the inverse quantization unit 6, the inversely quantized signal being inputted into the inverse DCT unit 7 and inversely DCT-processed to thus be restored to the original signal. The restored signal is added to a signal selected from a second switching unit 11 in an adder 8 to be stored in the video memory 10. The second switching unit 11 selects the appropriate signal between the signal selected from the first switching unit 2 and the signal outputted from the video memory 10 to thus be outputted to an adder. The video memory 10 outputs a motion compensated video signal to the subtractor 1, and outputs a motion vector information(v=MV;motion vector).
However, in case of compressing a moving picture using the hybrid motion compensation discrete cosine transform technique, a blocking effect and ring effect occur in the video signal reconstructed at an video decoding unit due to the loss of information generated in the quantization process.
To eliminate the blocking effect in the video signal, a loop filter 9 is inserted between the adder 8 and video memory 10 of the video encoding unit. The loop filter 9 reduces the blocking effect by searching for the motion vector information of the filtered video signal.
In case of transforming the video signal in order to search for the motion vector information of the video signal, the video can be taken throughout the entire video, or can be taken by dividing the same into block units of a predetermined size.
However, in a case that the transform and quantization process is performed in block units of 4×4 size, a blocking effect occurs apparently at a low transmission bit rate, though a blocking effect and a ring effect are reduced as compared to the conventional art
In addition, in the quantization process, as the value of the size of quantization becomes higher, compression efficiency becomes higher. However, because the loss of information is increased, a phenomenon that an video is visually unclear is occurred.
This phenomenon can be described by classifying the same into a blocking effect and a ring effect by using the hybrid MC/DCT compression method.
The blocking effect is caused by the result that the entire video is shown as being divided into blocks since the video signal is divided into a predetermined size and each block is independently processed, and by the loss of information of a low frequency region occurring in the quantization process.
The ring effect is caused by the loss of high frequency information due to the quantization process after block transform, which occurs since the contour line of a moving picture is formed in a water waver shape.
Among a number of methods for eliminating such a blocking effect or ring effect, the H.26L standardized compression method developed in the ITU-T is recently used. In the H.26L standardized compression method, a blocking effect is eliminated by using the loop filter in the video encoding unit and the video decoding unit, and a blocking effect or ring effect is eliminated that has not been processed completely in the loop filter by using the post filter. In the H.26L standard method, unlike the conventional compression algorithm, an video is processed in block units of 4×4 size, and a variable block unit motion compensation method is used. In addition, in the quantization and inverse quanatization method, changes in size of quantization is nonlinearly defined, unlike the conventional method.
However, this method is inefficient because the loop filter is used only for an intra frame, the video signal restored after post-filtering is redundantly smoothed, and a prediction residual between the video signal of which the motion is compensated by the loop filter and the original video signal is increased.
To solve the above problem, the low pass filtering(LPF) method and the normalization restoration method are proposed.
The low pass filtering method is a method for reducing a blocking effect and a ring effect by obtaining a pixel value according to a designated filter coefficient value by selecting a plurality of pixels around a certain pixel.
However, the low pass filtering method has a problem that the quality of the restored video signal is redundantly smoothed according to the type and compression factor of the video signal, since the value of a filter tap or filter coefficient for eliminating a blocking effect is set according to an experimental value.
The normalization restoration method is a method for eliminating a blocking effect by obtaining the statistical characteristics of an video signal by calculating the value of marginal portions at four sides of a block and the degree of non-uniformity in the block. In the normalization restoration method, there is difficulty processing in real time, because calculation is performed on each video signal, and the equation for the calculation has a matrix form, thus requiring a large computational complexity. In addition, when calculated normalized coefficients determine the degree of smoothness, the average value is applied irrespective of the degree of non-uniformity, thereby reducing adaptability.
For example, in a case that the normalized coefficient value applied to a region having a serious blocking effect or a region having a reduced blocking effect, the region having a reduced blocking effect is redundantly smoothed.